This invention concerns an image reading device used in electronic print boards, facsimile machines or the like, in particular a device capable of detecting that a light source for illuminating a source document is off.
A conventional image reader of this type is shown in block diagram of FIG. 1. In this figure, 1 represents a lamp for lighting a source document P, 2 represents a shading plate smooths the light intensity distribution of a reflected light L from the source document P, 3 is an image formation lens, and 4 is a line sensor which includes a line of photoelectric conversion elements for producing an image signal A. 5 is an amplifier for amplifying the image signal read by line sensor 4 to produce an amplified image signal B, 6 is a comparator which compares the amplified image signal B with a slice level D to produce a series of binary pixel signals, 7 is a counter for counting the binary signals of a value "1", 8 is a control circuit, 9 is a memory, and 10 is a D/A converter.
In the above configuration, the image signal A produced by line sensor 4 is amplified by amplifier 5, becoming the amplified image signal B, and is input into comparator 6. Image signal B is then converted to binary signals F by way of a comparison with slice level D produced by D/A converter 10.
Prior to the start of actual image reading, the peak valve of one line of the image signal is determined and a threshold level to be used in the subsequent actual image reading is obtained from the peak valve thus determined. This is done in the following manner. For purposes of the following description, "actual image reading" means the process of reading data from an area of source document on which images (characters, symbols, pictures etc. to be processed) have been written. The "threshold level" means the slice level D used during actual image reading. FIGS. 2A and 2B are graphs showing the relationship between the slice levels and the image signals. As shown in FIG. 2A, as a first step, control circuit 8 calls for the maximum value of the slice level information (these slice levels are used for finding the peak valve of the image signal) stored earlier in memory 9; then control circuit 8 inputs into comparator 6 the slice level information E, which is converted at D/A covverter 10, together with the reference voltage Vco, into an analog signal D of a value of slice level VP1 stored in memory 9 and under this condition, line sensor 4 reads one line of the image, and image signal B is input into comparator 6. The image signal B is compared with slice level VP1, and is output as binary signals F consisting of "1" and "0" bits. Binary signals F of a value "1" are counted by counter 7; the result of the counting, G, is then input into control circuit 8. When the count value G at the end of reading of each line is less than a certain value, control circuit 8 calls from memory 9 slice level information, e.g., VP2, which is one step lower than the previously called slice level information, e.g., VP1. In this way, the control circuit repeats the same operation, gradually decreasing the slice level information until the count value G becomes equal to or greater than the defined value. Suppose, for example, that the slice level VPj is the largest slice value which gives the count value G of not less than the defined value. Then the slice level VPj is recognized as the peak level and control circuit 8 raises a carry flag, calls from memory 9 the threshold level SLj information corresponding to peak level VPj, sets SLj as threshold level D, and then performs actual image reading.
In such a configuration, when lamp 1 is not lit or off the carry flag will not be raised by control circuit 8 even if the slice level is lowered to VPm, the minimum slice level obtainable from the slice level information stored in memory 9. When this happens, the conventional system would conclude that lamp 1 is not lit.
Another example of a conventional system is the configuration shown in the block diagram of FIG. 3. The system in this example differs from that in FIG. 1 in that it has peak-hold circuit 11 for holding the peak value produced by amplifier 5, and that it varies reference voltage V.sub.VAR input into D/A converter 10 according to the output from peak-hold circuit 11. This enables the system to lower the threshold level when the light emission intensity from lamp 1 diminishes due to aging changes or ambient temperature changes. This also enables the system to set threshold level D consistent with the level of image signal B to enhance the fidelity with which source document P is read.
The above conventional systems have the following problems. The conventional system of FIG. 1 is capable of detecting the condition of lamp 1 not lit. However, if the light emission intensity of lamp 1 changes due to the aging or a decrease in the ambient temperature, since threshold level D remains at a fixed level, threshold level D will become inappropriate, resulting in decreased fidelity of image reading relative to the source document image.
Also, even though it was possible for the conventional system of FIG. 3 to set a threshold level by taking into account the light emission intensity from lamp 1 to ensure good fidelity reading of images even in the event of a change in light intensity from lamp 1, when lamp 1 is not lit, reference voltage V.sub.VAR will decrease from the condition shown in FIG. 4A to that shown in FIG. 4B. When this happens, the level of image signal B (corresponding to the dark electric potential level) becomes larger than the lowest slice level VPm, as shown in FIG. 4C which is an enlargement of FIG. 4B in the direction of the signal level. Under this condition, the conventional system of FIG. 3 would fail to detect that lamp 1 is not lit.